Process for separating uranium fission products



March 10, 1959 F. H. sPEDDlNG ETAL 2,877,109

PROCESS FOR SEPARATING URANIUM FISSION PRODUCTS O Filed April 12. 1945 Qm @wg United States Patent O PROCESS FOR SEPARATING URANIUM FISSION PRODUCTS Frank H. Spedding and Thomas A. Butler, Ames, Iowa, and Iral B. Johns, Santa Fe, N. Mex., assignors to the United States of America as represented by the United States Atomic Energy Commission Application April 12, 1945, Serial No. 588,054

12 Claims. (Cl. 75-84.1)

Our invention relates to the purification 'of neutron irradiated uranium metal and more particularly to the removal of highly radioactive substances from neutron irradiated uranium. y

The term element 94 is used throughout this specication to designate the element having atomic number 94. The designation 94239 refers to the isotope of element 94 having a mass number of 239. Element 94I is also referred to in this specification, and probably will become known in the art, as plutonium, symbol Pu. Likewise, element 93 means the element havingatomic number 93, which is also referred to as neptunium, symbol' Np. Reference herein to any of the elements is to be understood as denoting the element generically, whether in its free state, or in the form of a compound, unless' Votherwise indicated by the context.

v Natural uranium comprises largely isotope U239, together with about M39 as much U335 and a very much smaller amount of U234. When this or other mixture ofthe isotopes U335 and U339,'either as metallic uraniumkor as a uranium compound, is subjected to bombardment by neutrons or undergoes a self-sustaining neutronic chain reaction, a number of nuclear reactions take place. Isotope U239 captures neutrons to form U23", which undergoes beta decay to form transuranic elements:

` 23 ruin. (B) 9333' 2.3 Days (B) 94239 'mas-.0111 Um Isotope U335 undergoes fission, i. e., a breakdown of its heavy nucleus into lighter fragments which are generally very unstable and highly radioactive. Such fragments usually undergo beta-particle disintegration in successive steps, leading ultimately to stable isotopes of higher nuclear charge than the original fragments. The lission of U235 is predominantly binary, and may be exemplified -by the following type of equation:

ua-tonl-Krw-l-Banl-i-xonl-l-175 m. e. `v. i

where x is a small number greater'than unity.. f

Substantially all of the fission fragments have mass.y numbers wtihin the range 77-158y although small:v

quantities of isotopes of lower 4and higher mass numbers may result from unbalanced'binary ssions, ternary ssions, or other reactions of infrequent occurrence. A very large majority of the fission fragments cornprises a light group of mass numbers 84-106 and a heavy group of mass numbers 12S-150.

The-various fission fragments and decay products of '2,877,109 Patented Mas. 1o, 195s ICC of years. The effect of those having very short-lives may be practically eliminated by aging the material for a reasonable period before handling. Those with very long half-lives do not have suiciently intense radiation to endanger personnel protected by moderate shielding. On the other hand, the fission productshaving'half-lives ranging from a few days to a few years may have dangerously intense radiations which cannot be eliminated by aging for practical storage periods. These products are chiefly radioactive isotopes of Sr, Y, Zr, Cb, and Ru of the light group and Te, I, Cs, Ba, La, Ce and Pr of the heavy group.

The total amount of 93239 and 94239 produced from U239 in natural uranium by neutron bombardment .is a function of neutron density and of the time of bombardment. Since 94239 is fissionable under the conditions for fission of U235, the net yield of 94339 per unit -of time will decrease as the ratio of U235 content to 94239 content of the mass decreases. For this reason,l a neutronic reaction for 94239 production is suitably terminated when only a fraction of the U239V has beencon verted to ission products. The reaction mass at this, point contains a large amount of U233, a much smaller amount of U235, still smaller amounts 0593239; 94339 and fission products, and traces of other products such as UX1 and UX2. By aging such a mass for asuitable period of time, the 93239 may bev substantially completely converted to 94239 or at least untilthe 94 content exceeds the 93 content of the irradiated product',I with simultaneous conversion of at least a portion of the short-lived radioactive fission products to longerlived or stable isotopes.

The plutonium content of the aged reactionbmassl will usually be considerably less than 1 .percent of the'.v

weight of the unreaeted uranium, and may even be less than one part by weight per million parts by weight-'of uranium. The concentration of ssion products will.

be of the same order of magnitude.

The problem of securing a satisfactory lremoval the fission products from the uranium "and thereby reducing the radio-activity thereof is difficult particularly because of the low concentrations of the fission products radioactivity of the uranium reducedjto a substantial degree from neutron irradiateduranium` by heating.,

neutron irradiated uranium in metallic state to a temper- 4ature above 1000 C., preferably l fusing the metal and removing the fission products or a portion thereof; In general the fission products are transferred to a dile'r-A ent phase and are removed in the form of sucha phase. For example numerous products suchv as radioactive barium, strontium, cesium, rubidium, iodine etc. may be vaporized from uranium metal atleast to an appreciable degree. Moreover if the uranium is maintained in molten-- state at the proper time gaseous decay productsof the: fission fragments such as rare gases including radioactive xenon will be released. t

In addition removal of certain of the fission products.

may lbe facilitated by introducing agents into the melt uoride or Carbon'itetrchloride for elemental .halogen suchas iodine, may be addedy to extract certain offthcfA radioactive isotopes by permitting stratificationv of thel y added halideY or-converting the isotopefitself toa volatile or more easily removable halide and permitting vaporiz'ation or separation thereof. Where the ssion products are removed by .aid of a Aslag forming agent such -as UF, or alkali metal or alkaline earth metal halides, the slag serves to extract the ission product from the uranium. Proper l precautions are generally observed to secure sufcicnt 'contact `between the slaggin'g component and the uraniumfas willbe understoodin the art.

In accordance with a further modication the uranium may -be lmelte'd with carbon, preferably below 1650 C. where the amount of uranium carbide formation is low, and Ainsuch `a case certain of the lission products are removed probably by formation of high melting point carbides `which separate as `a scum or slag or are absorbed or adsorbed Aby the carbon. Thus cerum, lanthanum, yttriuin and zirconium may be effectively removed when the uranium is melted in contact with carbon at a-tempe'rature -below that at which substantial uranium carbideis formed, preferably below 1650 C. This process usually results in formation of a slag comprising the carbides of `the impurities andthe purified metal may be tappedfrom beneath the slag and cast.

-Numerous other modifications may be resorted to. For example molybdenum and tellurium may be removed toan-"appreciabledegree by adding nonradioactive isotopes of these metals to the uranium and heating the mjolten mass in contact with limited supply of oxygen sulicient to form oxides of these agents without forma tion of excessive amounts of uranium oxide. In such a casethe `molybdenum and tellurium oxides formed are vaporized'and` removed.

another example a nonradioactive isotope of a ssionfproduct oran isotope of lower activity may be added andremoved by vaporization and/or slagging as herein contemplated. Thus nonradioactive, or slightly radioactive iodine, tellurium, barium, cerium or rubdium may be added and vaporized olf. In such a case a major portion of the mo're radioactive isotope of the same element is carried into vapor state and thereby removed thus decreasing `the radioactivity. Likewise molybdenum may be added, converted to a carbide and purified uranium removed Afrom `the carbide which may appear as a slag or scum on the molten metal or may be picked up by solid carbonv surfaces.

The invention may be more'fully understood by reference to the following description of certain embodiments `thereof -and to the Aaccompanying drawings `in which:

Fig. 1 is a diagrammatic view in side elevation, partially insection, of an induction coil in combination with a crucible containing irradiated uranium, and other associatedf'a'pparatus in which the present invention may be performed; land,

Fig. 2 js a diagrammatic cross sectional view of the chamber shown in Fig. y1 and showing the crucible partiallyin section.

`Radioactive impurities inY irradiated uranium may be oeor 'more of the following: bromine, krypton, rubidium, strontium, yttrium, zirconium, columbium, molybdenum,=g`adolinium, ruthenium, rhodium, antimony, tellurium, fiodine, xenon, cesium, barium, praseodymium, ne'odymium.

InFgs. 1 and2`the apparatus for practicing our invention comprises a graphite crucible 1` supported within a stand `2 on'a radial platform 2a, the stand Z being carried'byfa'cart `3. The space between the crucible 1 and the *stand 2 is filled with a suitable insulation `14 such as magesa, zirconia or other refractory insulating material (otshown). As a shield against .gamma ray radiation the cart 3 runs `on -a track 4 extending through concrete 'tre'n'ch into 'and out of a concrete pit 6 which is covered "by"a lead cover 7. The graphite crucible 1 is loadedwitha mass of irradiated uranium 8 and covered with acover 9 while standing in the trench 5, the cart 3 with a loaded cruciblel is run into the pit 6 for purication of the irradiated uranium Iby heating and melting `with consequent diffusion of the impurities. An lnducU tion coil 10 comprising hollow tubing and movable up and down on bus bars 11 attached to the side of pit 6 is designed to surround the stand 2 and crucible 1 and melt the uranium by induction when the cart 3 is positioned 1n the pit 6. The coil 10 slides up to clear the stand 2 and permit the cart 3 to `move in and out of the pit 6 for the purpose of loading andunloading the crucible 1. A current is supplied to the coil l10 and bus bars 11 through leads 12 which are attached to terminals on the bus bars 11. The leads 12 vrun from the pit 6 through the trench 5 to a suitable power source. The induction coil 10 is provided with a cooling system comprising flexible rubber tubes 13 connected tothe top and bottom of the induction coil 10 to supply a llow `of water which-runs through the hollow interior of the tubing of coil 10.

A mold 17 is placed on the cart 3 in a lower compartment 15 beneath the platform 2a and is so positioned that the molten uranium from the crucible 1 may be cast into it. The casting of the uranium into the mold 17 is accomplished by raising a plug 21 which seats in an aperture 22 in the bottom lof the crucible 1. The aperture 22 is set otf center "and has formed on its under or outer side a spout 23. In casting the uranium or raising the plug 21 the molten uraniumflows through theaperture 22 along `the spout 23 and into the mold 17. During the casting operation the plug 21 is raised by means of a rod 24 attached to theplug and pivoted on the end of a lever 25, the lever 25 extending through an opening 26 in the stand 2 to a point where it may be manipulated without danger tothe gamma radiation.

The preferred mode of purifying irradiated uranium will now be explained.

In the performance of the invention the crucible 1 and contents are covered by the cover 9 and the car 3 is then rolled into the pit 6 and positioned beneath the raised induction coil 10 as shown in Fig. 2. The induction coil 10 is then lowered over the stand 2 and crucible 1 into the position shown in Fig. `2 and the current turned on to heat the uranium mass generally to between 1200 C. and 1650" C. fusing 'the contents of the crucible. Where the crucible is made of graphite and/or where other slag forming ingredients are incorporated a slag of uraniumcarbon alloy or other slag is formed, the uranium-carbon alloy slag tending to line the inner surface of the crucible. The metal is maintained in molten state for a substantial period of time usually approximately 2 hours or until a suitable separation of phases has occurred. Thereupon the plug 21 is forced upward by the levers 24 and 25 to allow ow of the melt of purified uranium into the mold 17. The residue of the slag composed of the ux and the radioactive impurities clings to the wall of the crucible 1 and is thus separated from the purified uranium. The cart 3 is then drawn out of pit 6, the mold 17 removed for further working of the purified uranium and the crucible 1 is disconnected from'the levers 24 and 25 to be set aside for the radioactivity of the fission fragments in the slag to decay. A fresh crucible 1 and mold 17 are` placed in the stand 2 onthe cart 3 and the process is ready to be repeated.

In this 'apparatus the parts are inexpensive so that in thecase of a breakdown the entire unit may be aban doned until the decay of radioactivity has proceeded to the polnt where repair can be made. In the same way the parts as they become radioactive may be discarded or set v'aside to allow for radioactive decay and fresh parts substitutedfor them. Moreover the arrangement of the apparatus andtherprocedure of the process permits the .entire operation to'b'e conducted by remote control and thereby eliminate many of the dangers from gamma or other radiation.

The metal thus purified is found to be suiciently free from radiation and heating due to radioactive decay may be `more safelyhandled than the raw irradiated uranium.

Thus the processserves as an initial ltreatment of the highly radioactive irradiated uranium to reduce the radioactivity to safe or manageable limits. The radioactive impurities may have considerable commercial value as a by-product and by theiry absorption in the uranium carbide or similar slag they are concentrated in a small volume of material. The radioactive elements are easily handled and vstored in this medium and mayv be removed from the uranium carbide without having to work up the bulk of raw irradiated uranium thus making the recovery easier. The radioactive gases which evaporate from the fused irradiated uranium may be collected on charcoal if it is desired to eiect their recovery or prevent their escape. The following examples are illustrative:

Example 1 Samples of neutron irradiated uranium containing several parts per million of fission products were heated in graphite crucibles at temperatures from 600 C. to l800 C. for a period of about two hours and the amount of removal of the iission products determined by analysis. The following table shows the amount thus removed in percent of the amount of the radioactive isotope initially present.

About 70 to 80 percent of the total gamma activity was thus removed. Further removal may be eiiected by repeating this process.

Example 2 150 grams of neutron irradiated uranium which had received 53,000 microampere hours of neutron bombardment was heated in a graphite crucible to 1400 C. and a mixture of argon and nitrogen bubbled through the melt `until one liter of nitrogen had been introduced. The molten metal was drained from the slag and analyzed. Fission products were removed as follows:

Element: Percent removed Ba 95 Sr 95 Te 66 Ce 98.7 Zr 7 1 Rare earths 97 The removal of tellurium, cerium and rare earths was substantially higher than when no nitrogen is used.

Example 3 A weakly bombarded sample of metal was run using 5 grams CaFZ per 100 grams of metal as a ux for the purpose of determining the eiect of such a ux in removing the lission products. The tlux was placed in the bottom of the crucible, the uranium placed on top and the mass melted. The molten massy was heated for one hour at 1400 C. in a graphite crucible and a slag layer separated from the metal. The purified metal was tapped from the bottom of the crucible and cast. Upon examination of the cast metal it was found that the total gamma activity of the metal was reduced 74.25% when measured through 0.739 gJcm rb; 'The activities-were divided about equally between the iiux and the carbide. The use of the ux appeared to improve the quality of the recast.

Example 4 The process lof Example@ lwasr'cpe'ated? using UF4 in lieu CaF2. After heating for 2. hours the metal was cast and sampled. Then it was remeltedto 1400 C. and carbon tetrachloride-argon mixture bubbled through the melt for about 2 hours. The metal was maintained at 1400 C. for 2 more' hours in the absence of air and in a graphite crucible. The following"table indicates the results:

Remaining Percent Before C014 Remaining Element Treatment but Percent Alter After UF4 4 Treatment Treatment Radioactive Xe-. 0. 006 None found. Radioactive 1-..- 0.0 Do. Radioactive Ba 2 Do. Radioactive Sr 2 Do. Radioactive Mo.--. 94 68. Radioactive Te- 69 61. Radioactive Ge 1. 4 0.3. Radioactive La and Y 1. 9 None found Radioactive Zr 2. 5 Do. Radioactive 93 90 90. Radioactive 94 90 90. Radioactive Ux 12. 8 None found.

It will be apparent to those skilled in the art that various modifications may be made without departing from the principles of the invention as disclosed herein and for that reason it is not intended that the invention should be limited other than by the scope of the appended claims.

Having thus described our invention the following is claimed:

What is claimed is:

l. A process for separating radioactive fission products from metallic uranium containing said elements in a mixture, comprising adding a reagent yfor said elements to said mixture, said reagent being selected from the group consisting of uranium halide, and carbon; melting said mixture while in contact with said reagent; maintaining said contact for a sutiicient time for some of the radioactive iission products to react with the reagent; and separating the compounds formed of said elements from the metallic uranium.

2. The process of claim l wherein the mixture is held at a temperature of about 1400 C. during reaction.

3. The process of claim l wherein the reagent is uranium bromide.

4. The process of claim 1 wherein the reagent is uranium fluoride.

5. The process of claim 4 wherein the melted reaction mixture, after reaction at about 1400 C. for about two hours, is treated with a CCL-argon current for about two hours and thereafter a temperature of approximately 1400 C. is maintained in the absence of air for another two hours.

6. The process of claim l wherein the reagent is carbon, the reaction temperature approximately 1400 C. and a mixture of nitrogen and argon is bubbled through the molten mass during reaction.

7. The process of claim l where the reagent is carbon and the melted mixture remains in contact therewith for a sui`cient time to form a carbide of one or more of the radioactive elements in the mixture.

8. The process of claim l where the reagent is carbon and the melted mixture remains in contact therewith for about two hours.

9. The process of claim 7 where the temperature is maintained at about 1400 C.

10. The process of claim 7 where a mixture of argon and nitrogen is bubbled through the mixture.

fl `1 1. .The Aproemls 'of claim "8 :Where the tempiexfature'is REFERENCES maintained at bout 1400? ic' V I `Moo;e: PreparationbfMetllic Uranium, Transac- The MP'IOCCSS "0f cla-1m i1 Where the 'reagent 1S a tions'o'f"the 'American *Electrochemical Society, 1vol. 43, famum hallde pp. 317-328, parficulaly page 326 (1923).

Mellor: Comprehensive Treatise of Inorganic `and Reference Cied'n *hlwfhs Patent Theoreiicn Chemistry, v01. l12, p. 11 (1932), also page UNITED `STKITS PTENTS 10. 1,437,934 Mal-den Dea ,5, 1922 Seaborg: y Chemical` and Engineering News, vol. 23, 1,563,685 Meere Jan. `5, 1926 w N0 23111-2192 (`1945) 1,728,942 Marden Sept. 24, 1929 2,085,450 Rohn June29. 1937 

1. A PROCESS OF SEPARATING RADIOATIVE FISSION PRODUCTS FROM METALLIC URANIUM CONTAINING SAIC ELEMENTS IN A MIXTURE, COMPRISING ADDING A REAGENT FOR SAID ELEMENTS TO SAID MIXTURE, SAID REAGENT BEING SELECTED FROM THE GROUP CONSISTING OF URANIUM HALIDE, AND CARBON; MELTING SAID MIXTURE WHILE IN CONTACT WITH SAID REAGENT; MAINTRAINING SAID CONTACT FOR A SUFFICIENT TIME FOR SOME OF THE RADIOACTIVE FISSION PRODUCTS TO REACT WITH THE REAGENT; 